Catalytic domain of the Protein Serine/Threonine Kinase, Mitogen-Activated Protein Kinase Kinase Kinase. Serine/threonine kinases (STKs), mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) subfamily, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The MAPKKK subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. MAPKKKs (MKKKs or MAP3Ks) are also called MAP/ERK kinase kinases (MEKKs) in some cases. They phosphorylate and activate MAPK kinases (MAPKKs or MKKs or MAP2Ks), which in turn phosphorylate and activate MAPKs during signaling cascades that are important in mediating cellular responses to extracellular signals. This subfamily is composed of the Apoptosis Signal-regulating Kinases ASK1 (or MAPKKK5) and ASK2 (or MAPKKK6), MEKK1, MEKK2, MEKK3, MEKK4, as well as plant and fungal MAPKKKs. Also included in this subfamily are the cell division control proteins Schizosaccharomyces pombe Cdc7 and Saccharomyces cerevisiae Cdc15.